1. Field of the Invention:
This invention relates to a pneumatic snow tire having a tread superior in performance on snow covered roads and more particularly to an improvement of a radial tire for a snowmobile.
2. Description of the Prior Art:
The term "snow tire" used herein is intended to designate a tire to be used for vehicles such as snowmobiles traveling on snow roads.
Pneumatic tires for snowmobiles have been known, whose tread includes two or three circumferential zigzag grooves equally spaced apart from one another and a number of bias or transverse grooves extending from corners of the zigzag grooves and connecting between the zigzag grooves and between them and tread edges to form blocks divided by these grooves. In order to obtain driving and braking forces and a steering stability in traveling on snow roads, the tire for a snowmobile has a tread including circumferential zigzag grooves whose amplitudes in width directions of the tread are much larger than those of general tires, so that blocks formed between the zigzag and bias or transverse grooves include comparatively sharp corners extending at acute angles adjacent to retracting corners of the zigzag grooves.
It has been found in inventor's experiments on such conventional tires for snowmobiles that extending the corners of the circumferential zigzag grooves in the tread surfaces contacting a road during travel causes bending deformations of the tread and the deformable sharp corners of the blocks tend to collapse the snow (snow columns) caught in the grooves so that expected high shearing forces in the snow at the tread surface cannot be effectively utilized. Such disadvantages are enhanced by the depths of the grooves which are much deeper than those of general tires. Moreover, such conventional tires are apt to clog their grooves with snow or the like and produce high noise during travel.